Ok, I have an M801 in my hand. This is simply 'out of the box', not a review, I'll post a video review of it over the weekend, once I get some free time as well as some time with it.
It's the same diameter as the M40*/EVO/Pilot, and indeed, uses the M401/2/3/EVO/Pilot carts, and looks like a Smoore unit..which is good news, as Smoore IMO makes some of the best e-cigs out there reliability-wise.
It's a few mm longer than an EVO/M402, but the battery length is closer to, or the same as, an M401 (need to dig out my M401 batteries for exact measurement, will before the review, have stuff packed up as I'm rebuilding the home office..).
Draw is like the M401/2/3/EVO, that is, kinda of like an analog, not a super hard draw, not like sucking air through a straw, 'just right' or close to it in my book, for my preferences.
The atty wick looks like an EVO atty, perhaps even wider. I dig the USB wall charger - it's small but wide, and the prongs fold up into it.
It produces good vapor, and if there's a cutoff, it's at or over 10 seconds.
I'll withhold my opinion on throat hit, as it's a new atty, and I haven't spent more than a few hours with it yet, but it 'feels' like an M401/2/3/EVO, perhaps with a bit of vapor and/or throat hit difference.
The carts interchange, the batteries and attys do not, as the battery and atty is 'reverse gendered' versus the M401/2/3/EVO/Pilot.
Initial 'pick up and use it reaction' so far - I like it, it's like my Ms/EVOs. Some time with it will tell if it's better/worse/identical to my EVO and M401s functionally, but there's nothing I dislike about it compared to those on initial reaction.